Bending Light

Refractive error, or the degree from which you do not have normal
vision, is commonly measured in diopters and indicates the amount light bends within your eye to be focused on the retina and be "seen". A diopter is often represented by a capital "D" in
a prescription. No refractive error is referred to as "plano", often
represented as "pl". The greater the refractive error, the larger
the number for both sphere,
representing the amount of myopia or hyperopia, and and cylinder, representing astigmatism. This known as a spherocylinder or spherocylindrical
prescription.

Spherocylindrical Prescription

A typical spherocylindrical prescription would look like:

sphere

cylinder

axis

OD:

-2.75

-1.25

x15

OS:

pl

-0.75

x85

OD is an abbreviation for the Latin oculus dexter, meaning
right eye. OS is an abbreviation for the Latin oculus sinister,
meaning left eye. The first number after the determination of which
eye is the sphere. A negative number indicates myopia. A positive
number indicates hyperopia. The second number in this prescription
is the cylinder (astigmatism), and the third number is the axis
of the cylinder component. The axis of the astigmatism does not
relate to the amount of cylinder, just the location of the irregularity.
If the patient has no cylinder, then the last two columns may remain
blank, or "DS" for "diopter sphere" may be used.

This prescription shows that the patient has 2.75 diopters of
myopia with 1.25 diopters of astigmatism at an angle of 15 degrees
in the right eye, and the left eye is plano with 0.75 diopters of
astigmatism at an angle of 85 degrees.

sphere

cylinder

axis

OD:

-2.75

-1.25

x15

OS:

pl

-0.75

x85

The above prescription shows that the patient has 2.75 diopters of
myopia with 1.25 diopters of astigmatism at an angle of 15 degrees
in the right eye, and the left eye is plano with 0.75 diopters of
astigmatism at an angle of 85 degrees.

Minus Cylinder or Plus Cylinder

Spectacle prescriptions can be written in two value sets, minus
cylinder or plus cylinder, which are mutually exclusive of each
other but provide the same information. As a rule, ophthalmologists
write scripts in minus cylinder whereas optometrists write scripts
in plus cylinder. Why these two professions cannot get together
and decide on a common method of reporting refractive error is impossible
to explain, but if you have ever had competing siblings in your
family, you may get an idea why this has not yet occurred.

To convert a minus cylinder form prescription into plus cylinder,
or to convert the plus cylinder form into minus cylinder, do the
following:

Add the sphere and cylinder powers together; this becomes
the new sphere power.

Change the sign of the cylinder power, from minus (–) to plus
(+) or from plus (+) to minus (–).

Change the axis value by 90?, remembering that the axis must
be a number from 1 to 180.

The following lens prescriptions, therefore, are equivalent and
interchangeable:

These two prescriptions
are exactly the same
presented in both minus cylinder and plus cylinder form.

sphere

cylinder

axis

sphere

cylinder

axis

OD:

-2.75

-1.25

x15

OD:

-4.00

+1.25

x105

OS:

pl

-0.75

x85

OS:

-0.75

+0.75

x175

minus cylinder form

plus cylinder form

Spherical Equivalent

The spherical equivalent power of a lens prescription is the
average of the dioptric powers in all meridians of a lens. To obtain
this value, add half of the cylinder power to the sphere power.
In other words, do the following:

Divide the cylinder power by 2.

Add this value to the sphere power; the result is the equivalent
sphere power of the lens.

For the glasses prescription...

sphere

cylinder

axis

OD:

-2.75

-1.25

x15

OS:

pl

-0.75

x85

minus cylinder form

...the equivalent sphere powers of each lens would be calculated
as follows:

Abstract
PURPOSE: To evaluate refractive outcomes of toric intraocular lens (IOL) implantation with a detailed analysis of decentration and its effect on aberrometry.
METHODS: This retrospective study enrolled 19 eyes implanted with SN60T AcrySof(®) Toric (Alcon - USA) IOL's. Spherocylindric correction was studied in depth by the Alpins method, and retro-illumination images were used to analyze the alignment of the IOL with its intended axis. IOL decentration as well as its aberrometric impact were evaluated with a strict, novel protocol.
RESULTS: Three months postoperatively, uncorrected distance visual acuity was greater or equal to 20/40 in 94.74% of cases, postoperative subjective cylinder was less or equal to 0.5 D in 68.42% of eyes, with a mean index of success of 0.24. Mean error of toric IOL alignment was 5.68° (0 to 14). Mean IOL decentration was 0.78 mm (0 to 1.78) with a mean coma and trefoil of 0.18 μ (0.06 to 0.33) and 0.19 μ (0.05 to 0.51), respectively. The larger the IOL decentration, the higher the optical aberrations were.
CONCLUSION: Toric intraocular lens implantation is an effective, safe and predictable method of spherocylindrical correction during cataract surgery, with a refractive accuracy similar to that of LASIK in the treatment of astigmatism in young patients. IOL decentration produces optical aberrations including coma and trefoil, which interfere with visual performance.

PMID: 23332291 [PubMed - indexed for MEDLINE]

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